The invention concerns a micropolarimeter comprising an analyzer and a detector located past the analyzer and presenting a number of segments N.sub.0.
In the state of the art, light is used as a contactless and non-destructive probe for measuring a wide variety of sample properties. For this purpose the change of the characteristic properties of the radiation after interaction with the sample is used; for example, reflection or transmission. In contrast, in polarimetry and ellipsometry, the information contained in the polarization properties or its changes due to interaction with the sample is used.
In the case of completely polarized light, this information is the ellipticity, the position of the major axis of the polarization ellipse in space (azimuth) and the sense of rotation of the field strength vector. In the case of partially polarized light, the degree of polarization is added. These quantities are measured completely by the four elements of the Stokes vector, the so-called Stokes parameters (see R. M. Azzam, Bashara, Ellipsometry and Polarized Light, North Holland, Amsterdam, 1988). Devices for measuring the parameters of polarized light are called polarimeters. A wide variety of measuring tasks may be performed by means of completely polarized light whereby the sense of rotation of the field strength vector does not need to be known. In this case, it is sufficient to determine ellipticity and azimuth.
To determine components of the Stokes vector, devices with mechanically moving analyzers or phase shifters (retarders) are frequently used. Systems with moving parts always have the disadvantage of being slow, susceptible to malfunctions, expensive and relatively large. This is particularly disadvantageous in process control applications: high measuring accuracy, reliability, cost efficiency and flexibility play an increasingly important role in the installation in production lines. EP 0 632 256 A1 and U.S. Pat. No. 5,502,567 therefore proposed a very small, very fast polarimeter without moving parts. It is based on a polarizing film system, which is deposited on the surface area of a glass cone. The coated cone is then arranged along the optical axis of the system via a circular detector array to produce a unique allocation of angle sectors of the polarizing cone and the detector elements. This device is very small, avoids any movement and can be read out very quickly due to its parallel mode of operation.
A disadvantage, however, is the strict monochromasy of this cone polarimeter because the polarizing film system is a narrow-band filter that has the desired selectivity for polarization only in a very narrow spectral range. Spectral applications or a change to other wavelengths than the design wavelength are thus excluded.
A further disadvantage is the technically complex production of the polarizing film system. Coating, for example, must take place at a very steep angle. Furthermore, very high homogeneity of the film parameters on the surface area is required. These requirements cannot be satisfied with standard coating lines, which are laid out for vertical coating geometries.
A further disadvantage is that this device permits only two polarization parameters (ellipticity and azimuth) to be measured. To determine the complete set of Stokes parameters requires additional measurements with the inclusion of retarders, as discussed, for example, in P. S. Hauge "Recent Developments in Instrumentation in Ellipsometry." Surface Science, 96 (1989) 108-140.
Furthermore, EP 0 632 256 A1 and U.S. Pat. No. 5,502,567 propose a variant for miniature polarimeters based on a circular arrangement of metal grids. In practice, only polarization degrees of 3 were obtained. These low values are not sufficient for applications in quantitative polarimetry where components with polarization degrees&gt;10,000 are typically used to achieve high measuring accuracy.
The cause of the relatively low degree of polarization in grid polarizers is the selective reflection of the components of the electric field oscillating parallel to the grid lines. Since this is thus a surface effect, only a finite degree of polarization can be achieved per unit area. At a wave length of 670 nm, for example, a theoretically obtainable degree of polarization of approximately 100 was determined for a grid with a grid constant of 100 nm. Such a grid, which is produced by electron lithography, supplies practically only values of 2 to 3 (see B. Stenkamp et al, "Grid Polarizer for the Visible Spectral Region."SPIE Proceedings Vol. 2213).
The theoretically possible polarization degree of grid polarizers largely depends on the proportion of the grid constants to the wavelength. For an optimum effect, the wavelength should be greater than the grid constant. In the visible spectrum, this means grid constants of less than 100 nm, which are difficult to produce. Thus, to realize this device requires the use of electron beam lithography in conjunction with highly complex thin film processes. Both are connected with very high costs. Until now, such grid polarizers have therefore been produced primarily for applications in the infrared region where conventional photolithography with a resolution of approximately 1 .mu.m may be used for wavelengths of some 10 .mu.m to 100 .mu.m.
Furthermore, EP 0 632 256 A1 and U.S. Pat. No. 5,502,567 proposed a variant based on polarizing waveguides, which perpendicularly open onto the detector surfaces. This variant theoretically supplies a very high degree of polarization, but the disadvantage is the extremely low transmission of this device, which is to be expected due to the low degree of area filling and the high coupling losses connected with coupling the wave into the waveguide. U.S. Pat. No. 4,158,506 discloses a device with 6 rectangular analyzer elements arranged side by side with predetermined polarization direction, with 2 of the elements occurring twice and furthermore being provided with a .lambda./4 plate. The principle on which the analyzer elements are based, whether they are, for example, grids or the like, is not specified.
The asymmetry of the device, however, has serious disadvantages in practical application. For applications in ellipsometry, well collimated beams are used. The intensity distribution, however, is always in-homogenous. It typically fades toward the outside and is frequently Gaussian in shape. If such a beam strikes the 6-segment arrangement proposed in U.S. Pat. No. 4,158,506, the resulting signal distribution in the detectors depends not only on the polarization state but also on the position of the axis of the beam at the surface of entry. This means that those detector elements that are near the beam axis, receive greater intensity than other more remote detector elements. The analysis methods proposed in U.S. Pat. No. 4,158,506 therefore lead to large systematic errors. The only practicable option is to use an additional lens to expand the beam far enough so that only a very small and thus largely homogenous segment of the crest of the beam axis is reproduced on the device. This is connected with additional complexity and a significant loss in intensity, however, which is justified only in very powerful impulse lasers so that applications of this device have not become known thus far.
DE 44 42 400 A1 discloses a sensor to determine position in space, which has a cone that refracts a beam and a cylindrical transition element that is arranged on a detector array. The cone may be provided with an interference film system to separate the beam from parasitic radiation. The polarization of the beam cannot be measured with this device.